A method and a device for controlling a recirculating pump in a brake system are known from International Patent Publication No. WO 94/07717, which describes a method and a device for regulating the flow rate of a hydraulic pump which is driven by an electric motor with a variable pulse-pulse pause sequence. Such hydraulic pumps, which are also known as recirculating pumps, are used to produce an auxiliary pressure in a brake system having traction control and/or an anti-skid system.
During the pulse pauses, the generator voltage generated by the pump motor is analyzed as a measure of the pump speed; a pump speed setpoint is compared with an actual value of the pump speed in a control circuit, and a new manipulated variable for the pump control is derived from the difference. One disadvantage of this known arrangement is that it requires a very demanding controller which generates a pulse width-modulated signal on the basis of the difference between this setpoint and the actual value.
German Patent No. 44 40 517 describes a method of controlling a recirculating pump in a brake system, where the recirculating pump can be controlled by demand as a function of the value of a manipulated variable and/or a control deviation of a controller which influences the braking torque and/or the wheel rpm. This method also involves a relatively complicated control.
With electrohydraulic brake systems in particular, a hydraulic accumulator is usually charged by an electric motor and a high-pressure pump driven by the latter. At the start of control, the pump motor is controlled fully for a fixed period of time, i.e., continuously or uncycled. The electric motor driving the pump is connected to the on-board voltage of the motor vehicle. On reaching the desired pressure in the hydraulic accumulator, the pump can either be shut down or driven in cycled operation to maintain the pressure in the hydraulic accumulator. A pulse-pulse pause ratio of the pump drive is varied here as a function of a pump voltage applied to the pump. An initial pulse-pulse pause ratio at the start of cycled operation (i.e., after the end of continuous pump control) is preselected here as a constant. The duration of continuous pump control is also constant with traditional systems. However, since the on-board voltage in a motor vehicle can fluctuate greatly, these constant settings for various on-board voltages lead to different rotational speeds of the pump. For example, at a low on-board voltage, a pump can be brought to a certain set speed only by varying the pulse pause ratio accordingly during cycled operation. Consequently, a relatively long period of time is needed to establish a set speed because the pulse-pulse pause ratio is adjusted or regulated only after the start of cycled control.